According to a conventional thermal printhead of planar type, as shown in FIG. 1, the thermal printhead 4 opposes printing paper 2, supported against a platen 1, with its major surface, interposing a thermal transfer ink ribbon 3 therebetween. The thermal printhead 4 comprises a planar base plate 4a which is made of, for instance, alumina or other ceramic material, and a heat generating portion 4b formed on the major surface of the base plate 4a in a substantially semi-cylindrical shape. By selectively heating the resistive elements of the heat generation portion 4b by conducting electric current thereto and sliding the heat generation portion 4b over the surface of the printing paper 2 while interposing the transfer ink ribbon 3 therebetween, some of the ink in the ink ribbon 3 is deposited onto the printing paper 2 according to the distribution of heat in the heat generating portion 4b, and a desired print pattern is formed on the printing paper 2. However, if the surface texture of the printing paper 2 is excessively coarse, the contact between the ink ribbon 3 and the printhead 4 becomes insufficient, and the print quality is impaired because the ink would not be transferred from the ink ribbon 3 to the printing paper 2 in some places.
To overcome this problem, it has been proposed to use an end-surface type thermal printhead, having a heat generating portion 4b at an end surface of a base plate 4a, as shown in FIG. 2. FIG. 3 shows an example of such an end-surface type thermal printhead which was disclosed in Japanese patent laid-open publication No. 60-21263. According to this prior invention, the thermal printhead 4 comprises a ceramic base plate 11 and a glaze layer 12 formed on the end surface 13 of the base plate 11; the thickness A of the central part of the glaze layer 12 is selected to be approximately 80 micrometers. The thickness T of the base plate 11 is selected to be from 1 to 2.5 mm to give the surface of the glaze layer 12 a radius of curvature of from 2 to 10 mm. Such a glaze layer can be readily produced by printing a glass frit layer over the end surface of the base plate 11 or placing glass frit in sheet form over the end surface and baking it, by virtue of the surface tension of the molten glaze material tending to form a curved surface as determined by the material properties of the molten glaze layer and the thickness of the base plate. Thus, the thickness of the base plate is an important factor in determining the surface contour of the glaze layer 12.
However, according to the experiments conducted by the inventors, it was discovered that the radius of curvature R of the surface of the glaze layer 12 should be less than 2 millimeters and the thickness A of the central part of the glaze layer 12 is required to be less than 40 micrometers in order to obtain a satisfactory thermal responsiveness and a high print quality even when relatively rough paper is used. To achieve this goal, the thickness of the base plate is required to be less than one millimeter, and the mechanical strength of the base plate would be insufficient.
Furthermore, an end-surface type thermal printhead tends to cause a problem in the movement of the printhead relative to the paper particularly when the radius of curvature of the heat generating portion is reduced, and the formation of fine print elements in the heat generating portion which is located in a very narrow end surface of the base plate presents some problem to the efficiency of the manufacturing process.